The conventional Bourdon gage comprises a main body, a tubular mount formed with a male tread therearound and outwardly projecting from said main body and a Bourbon tube contained within said main body and connected to said mount. To measure a secondary pressure of a pressure control valve, for example, the main body itself is rotated to engage said male thread of said mount with a female thread of a gage port which communicates with a flow channel on the secondary side and thereby to achieve installation so that fluid on the secondary side may be introduced through said mount into the Bourdon tube and thereby the secondary pressure may be measured. In such gage, the gage itself may often interfere with its environment, making a smooth rotation of the body and therefore a proper installation if a space available around the gage port is excessively limited, since said installation is, as above mentioned, achieved by rotating the body itself so as to engage the male thread of the mount with the female thread of the gage port. Certainly it will become possible to achieve the proper installation when a length of the mount is increased, but this countermeasure will inconveniently result in a substantial projection of the gage itself outwardly from the main body of the pressure control valve.